1. Field of the Invention
This invention relates to an optical receptacle, and more particularly to an optical receptacle of the type wherein an optical connector plug at an end of an optical fiber is inserted into a hole of a receptacle body so as to be coupled to light emitting means or light receiving means.
2. Description of the Related Art
Various optical receptacles of the type mentioned are conventionally known. An exemplary one of conventional optical receptacles is shown in FIG. 1. Referring to FIG. 1, the optical receptacle 1 includes a receptacle body 2 in which a bore or axial hole 3 for receiving a ferule not shown of an optical connector plug at an end of an optical fiber therein is formed over the overall axial length. A thread 4 is formed at a rear portion of an inner periphery of the through-hole 3, and a cup-shaped threaded stopper 5 is screwed in the thread 4 to position the end face of the optical fiber and the end face of the ferule in the axial direction. Another thread 6 is formed at a front portion of an outer periphery of the receptacle body 2, and a cover not shown of the optical connector plug is screwed on the thread 6. When an optical module in which an optical semiconductor element, a lens and other elements are built in a package is to be mounted onto the receptacle body 2 to make an optical device, the threaded stopper 5 is removed and the package of the optical module is fitted into the through-hole 3 of the receptacle body 2 instead.
Where an optical fiber and an optical module are coupled in this manner, however, there is a problem that, since the end face of the optical fiber is a flat face perpendicular to the optical axis and besides is exposed as it is, the light transmitting characteristic of the optical device is deteriorated by reflection of light which occurs at the end face of the optical fiber. In particular, when the optical semiconductor element of the optical module is a light emitting semiconductor element such as a semiconductor laser, light converged by the lens of the optical module is partially reflected by the end face of the optical fiber and directed back to the light emitting semiconductor element to cause, for example, distortion of the waveform of a laser beam to be emitted from the light emitting semiconductor element.
On the other hand, when the optical semiconductor element of the optical module is a light receiving semiconductor element such as a photodiode, there is a drawback that also light propagating in the optical fiber is partially reflected by the end of the optical fiber and directed back into the optical fiber.
Several means for preventing reflection at the end face of an optical fiber have been developed and are disclosed, for example, in Japanese Patent Laid-Open Application No. 64-52103 and U.S. Pat. No. 4,790,618.
According to the means disclosed in Japanese Patent Laid-Open Application No. 64-52103, a glass plate having a refractive index equal to that of an optical fiber is interposed between the end face of the optical fiber, which is a flat face perpendicular to the axial line of the optical fiber, and a lens of an optical module. An anti-reflection film is coated on the entire surface of the glass plate except one face, and the face of the glass plate having no anti-reflection film thereon is held in direct contact with the end face of the optical fiber to prevent otherwise possible reflection at the interface between the face of the glass plate and the end face of the optical fiber. Meanwhile, at the other faces of the glass plate, reflection of light is prevented by the anti-reflection film.
With the means, however, since a high degree of accuracy is required for the installation of a glass plate, there is a problem that the operation for positioning and fixation of the glass plate is difficult. Further, the glass plate must be coated with an anti-reflection film except one face thereof, and accordingly, there is another problem that a high production cost is required.
Meanwhile, according to the means disclosed in U.S. Pat. No. 4,790,618, the end face of an optical fiber and the end face of a ferule are formed as inclined faces which are inclined with respect to a plane perpendicular to an axial line of the optical fiber, and a transparent cap having a refractive index equal to that of the optical fiber and having an outer diameter equal to that of the ferule is adhered to the inclined end faces of the optical fiber and the ferule. Further, an anti-reflection film is applied to the end face of the transparent cap.
With the means, however, it is difficult to adhere a transparent cap, which is small in size, to the inclined end faces of an optical fiber and a ferule with a high degree of accuracy and apply an anti-reflection film to the small end face of the transparent cap, and a high production cost is still required.